Alternatives to the Automobile

How Dangerous is Walking?

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In the last year or so, this statistic has been widely quoted:
>> Walking and cycling appear even more dangerous when these 
>> fatality rates are calculated on the basis of distance 
>> traveled.  Per kilometer traveled, walking is 36 times more 
>> likely to result in fatal injury than riding in a car...

I remember when the press release came out with those figures, in mid-2000. They were much discussed in car-free, transit, and "livable cities" type online groups.

I do have to wonder about that 36x statistic (walking being 36 times more dangerous than riding in a car, per km). Even using the auto-friendly RPM metric, I don't know how they came up with this figure.

To see what I mean, suppose we do a quick&dirty back-of- envelope computation...

Americans have collectively driven about 2.5 trillion miles per annum for the last several years. As of 1995 or so, at least, they walked a lot less than that :-)

A researcher at the University of California at Berkeley recently made a study of the nation's walking habits and found that the average person in the United States walks less than 75 miles a year -- about 1.4 miles a week, barely 350 yards a day.

That's as quoted by Bill Bryson in A Stranger Here Myself, circa 1995, so it's hardly a scholarly reference :-) But reflecting on the car-dependent lifestyle it doesn't seem impossible. We can take it as a postulate, even though the figure seems (to me) astonishingly low.

There are 285 million people in the US, I think. Say each person walks a mere 75 miles per annum: that's 21,375 million or 21 billion walking miles -- roughly 2 orders of magnitude (factor of 100) smaller than the number of miles driven/ridden.

So if risk were equal per mile, 100x as many people should die in cars as on foot, because their exposure is 100x greater. If walking is more dangerous than driving, then the ratio should be less extreme. If the same number of people died while walking as in cars, then walking would be 100 times more dangerous. Somewhere in between is our estimate of relative risk.

Each year about 35,000 people die inside cars, whereas 5,000 people die as pedestrians. That's a factor of 7, which goes into 100 about 14 times, so this back-of-envelope fatality risk assessment indicates that walking is 14x as dangerous as driving -- per mile, and disregarding any collateral effects like improved life expectancy from walking and diminished life expectancy from sedentary carcentric lifestyle.

14 is nowhere near 36.

So how did they come up with this statistic, I wonder?

We can hardly multiply up the VMT to account for it -- that's too well documented. So presumably we'd have to reduce the number of miles walked per annum by, umm, a factor of 2.6 -- so that would mean the average American only walked 29 miles per annum. That seems like an incredibly low figure: 135 yards a day? Wow . . .

Hmm, the average person walks about 3mph if I recall correctly, so 29 miles per annum means about 10 hours, or .0275 hours per diem spent walking. One tenth of an hour is 6 minutes, so .0275 hours is about 1/4 of 6 minutes, or 1.5 minutes. Is it really conceivable that the average American spends only 1.5 minutes a day walking? Please tell me it ain't true :-)

On the other hand, it's inconceivable that the authors of the much-cited study could have overstated annual VMT by the same factor: US VMT would have to jump to 6.5 trillion miles per annum! So it seems that a very low estimate of miles walked must be the key. 1.5 minutes a day . . . seems astonishing.

I use my bike most of the time and don't spend a whole lot of time walking (on public streets) but I still walk more than that in one year. I would think even the car-dependent would walk more than that, just getting to and from their parking spaces. That's one reason why senior faculty and staff agitated so strongly for a giant parking structure in the core of our campus -- they just hated having to walk from more remote parking lots :-) But it still has to be more than 1.5 minutes each way from the new structure to most of the surrounding buildings. That's already 3 minutes a day :-)

Anyway, one of these days I'll have to look up the original study and find out how the heck they got the 36x number. It was a very catchy number: within months of the press release, Woman's Day and other pop culture magazines were headlining the great dangers of walking. Another meme is born :-)
De Clarke